In general, natural rubber is known as the rubber exhibiting excellent mechanical properties, low hysteresis loss property and abrasion resistance. However, the processability is inferior to that of synthetic rubbers. This drawback arises since entanglements between rubber molecules increase due to the polypeptide bonds in the proteins present in non-rubber components of natural rubber latex used as the raw material and the apparent molecular weight increases to a great extent to increase the Mooney viscosity of the rubber.
To improve the processability of natural rubber, for example, in Japanese Patent Application Laid-Open No. Heisei 6(1994)-329838, a highly deproteinized natural rubber having a total nitrogen content of 0.1% by weight or smaller is described. Recently, various technologies for deproteinization have been proposed in the field of special applications such as natural rubber products for medical applications. Natural rubber from which non-rubber components such as proteins have been removed to a great extent are known (Japanese Patent Application Laid-Open Nos. Heisei 8(1996)-143606, Heisei 11(1999)71408 and 2000-19801).
However, although the natural rubber from which proteins have been almost completely removed exhibits improved processability, rubber compositions using such conventional deproteinized natural rubber have drawbacks in that modulus of the rubber decreases, the resistance to aging is inferior and the low hysteresis loss property is adversely affected since non-rubber components exhibiting the effects of antioxidation and vulcanization acceleration have been almost completely removed.
Moreover, the conventional deproteinized rubber has a further drawback in that the physical properties of the rubber markedly deteriorate since non-rubber components other than proteins are also lost when proteins are removed from a latex by centrifugation.
On the other hand, as the method for improving abrasion resistance of a tread rubber of a pneumatic tire, in general, it has been conducted that carbon black having a smaller particle diameter and a higher structure is used so that the reinforcing property of carbon black is enhanced by increasing the interaction between carbon black and the polymer or that carbon black is used in a greater amount.
However, dispersion of carbon black is poor in conventional rubber compositions containing natural rubber and carbon black having a very small particle diameter (a specific surface area by nitrogen adsorption (N2SA) of 80 m2/g or greater) and these compositions exhibit great viscosity and poor processability due to the poor dispersion. The physical properties such as abrasion resistance and the low hysteresis loss property (the low heat build-up property) are insufficient also due to the poor dispersion.
Dispersion of carbon black is poor in conventional rubber compositions containing natural rubber and carbon black having a low structure (a DBP absorption of 100 ml/100 g or smaller) and these compositions exhibit great viscosity and poor processability due to the poor dispersion. The physical properties such as abrasion resistance and the low hysteresis loss property (the low heat build-up property) are insufficient also due to the poor dispersion.
Rubber compositions containing silica have problems in that, in general, these compositions exhibit poor processability and, in particular, compositions containing natural rubber exhibit poor processability such as poor dispersion of silica and poor shrinkage of the obtained compounds. Therefore, sufficient physical properties cannot be obtained.
It has been difficult that excellent physical properties such as excellent abrasion resistance and low hysteresis loss property (low heat build-up property) are obtained by using rubber compositions containing carbon black or silica in combination with conventional natural rubber since dispersion of these fillers into natural rubber is poor.